Metastable Electron Compound in Ag-Ge Alloys

Abstract

The copper-silver system was chosen as a crucial test of the method because the existence of a eutectic is not in agreement with the Hume-Rothery rules, which would predict complete miscibility of the two metals. Four alloys containing 23.0, 39.9 (eutectic composition), 62.9, and 88.5 at. % copper were quenched from melt. The x-ray diffraction patterns of these alloys showed a single phase. On a plot of unit cell size vs concentration, the lattice parameters obtained with the four metastable alloys fell on a smooth curve connecting the two previously known segments of curves within the solubility limits at both ends of the phase diagram (14.1 and 95.1 at.% Cu). The maximum deviation from linearity (Vegard's law) was less than 1% at 50% Cu. Such a deviation is not unusual in solid solution binary alloys. These results establish without doubt that, during solidification, the two stable phases, copper rich and silver rich, did not have time to nucleate and grow, and metastable solid solutions were obtained. (Received February 19, 1960) I N the previous Letter to the Editor, 1 the existence of a continuous series of metastable solid solutions in Cu-Ag alloys was reported. By quenching the molten alloys rapidly enough, the normal processes of nucleation and growth were prevented and solid solutions were obtained. These metastable solid solutions were predictable since copper and silver satisfy the criteria for complete solid solubility proposed by Hume-Rothery.In Ag-Ge alloys complete solid solubility is precluded because of dissimilar crystal structures. The Ag-Ge equilibrium phase diagram 2 is of the eutectic type with a maximum solubility of germanium in silver of 9.6 at.% and negligible solubility of silver in germanium. The eutectic composition is at 25.9 at.% Ge. It was expected that the solubility limits on both ends of the phase diagram would be extended by rapid quenching. An alloy containing 25.7 at.% Ge was quenched from the melt. This alloy had a crystal structure that was neither face-centered cubic nor diamond cubic, but rather hexagonal close-packed with a=2.987±0.003, c=4.716±0.002, and c/a=1.628±0.003. The presence of a small amount of a diamond cubic gennanium phase was indicated by a few weak diffraction peaks on the x-ray pattern. The new metastable hexagonal phase is considered to be an electron compound with an electron concentration of 7/4 electrons/atom.In 1940, Hume-Rothery et al. 3 suspected the existence of a ! electron compound in Ag-Ge alloys, but failed to detect it under equilibrium conditions. In order to determine if such a compound could be synthesized by rapid quenching, an alloy containing 16.4 at.% Ge was investigated. No body-centered cubic phase was found, but again a hexagonal close-packed structure occurred with a=2.891±0.002, c=4.718±0.006, and c/a=1.632±0.003. The existence of the hexagonal structure instead of the body-. centered cubic structure at the electron concentration of ! has been noted in other alloy systems. Satisfactory explanations for the occurrence of a h...

Molecular-dynamics simulations of glass formation and crystallization in binary liquid metals: Cu-Ag and Cu-Ni

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Phase-field investigation on the non-equilibrium interface dynamics of rapid alloy solidification

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Morphologies of Microstructure in Mg<SUB>97</SUB>Zn<SUB>1</SUB>Y<SUB>2</SUB> Ribbon upon Ageing at Different Temperatures

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